This invention relates to a process for the preparation of uranium dioxide as well as to certain novel compositions of matter resulting from this process. Those concerned with the production of nuclear fuel have been constantly searching for processes for the production of actinide dioxides such as plutonium dioxide, neptunium dioxide and uranium dioxide which are cheaper, less complex, result in the production of the actinide dioxide in a pure state and further result in the production of ceramically active forms of the actinide dioxide (i.e., the physical form of the actinide dioxide must be such as to permit sintering thereof which is a necessary step in the preparation of pellets for use in nuclear fuel rods.
The present invention accomplishes all of the results set forth above by a process which permits the direct conversion of the actinide nitrate hexahydrate to uranium dioxide. An actinide nitrate hexahydrate such as uranyl nitrate hexahydrate is available commercially and is commonly produced during the process in which uranium is extracted from the ores, converted by a series of steps ultimately to "yellow cake" which consists principally of UO.sub.3, other oxides of uranium, and associated impurities. Following purification, usually by appropriate solvent extraction processes, the last step involves extraction of the uranium from an organic solvent into nitric acid solution; from this solution, pure uranyl nitrate hexahydrate is separated as a solid. This product is calcined to yield UO.sub.3, which is reduced to UO.sub.2 and thereafter fluorinated in two steps to obtain uranium hexafluoride. The uranium hexafluoride in turn is purified by successive distillations so as to yield a product with impurities in the parts per million range and the purified uranium hexafluoride may then be used in isotope enrichment processes, reduced with calcium to provide uranium metal of sufficient purity for use in the production of "weapons grade" plutonium or subjected to further purifications to result in the production of uranium dioxide in a degree of purity suitable for use as fuel for nuclear power reactors.
The present invention eliminates the need for many of the complex purification steps which would otherwise be required after uranium has been extracted from ores and has been converted to uranyl nitrate hexahydrate. If plutonium dioxide is available as the "enriched" fuel, the uranium dioxide produced by the present process may be admixed with the plutonium dioxide and thereafter used for nuclear fuel rods. Alternatively, only a small fraction of the enormous quantities of uranium hexafluoride presently processed need be purified and subsequently enriched in the U.sup.235 isotope (in the form of U.sup.235 F.sub.6 which is then converted to U.sup.235 O.sub.2) and the bulk of the U.sup.238 O.sub.2 required for nuclear fuel materials may be made by the process of this invention and thereafter admixed with the enriched U.sup.235 O.sub.2 material to obtain a blend suitable for use as pellets in nuclear fuel rods.
In recent years, efforts have been made to reduce the complexity of processes for the production of uranium dioxide in order to reduce fuel costs. For example, J. Belle ("Uranium Dioxide: Properties and Nuclear Applications," USAEC, 1961) atomized a solution of uranyl nitrate hexahydrate in the high temperature reducing atmosphere of a flame to produce uranium dioxide; however, this process provided no selectivity of reaction and impurity levels in the final product were essentially the same as in the feed liquor. Researchers at the Argonne National Laboratory, in 1963, converted UF.sub.6 directly to UO.sub.2 by a high temperature gas phase reaction of UF.sub.6 with H.sub.2 O and H.sub.2 but the resultant product still contained intolerable fluoride ion levels. In 1962, R. S. Wilkes (J. Nucl. Mat., vol. 7, page 157 (1962)) prepared uranium dioxide by electrolysis of a solution of uranyl chloride (UO.sub.2 Cl.sub.2) in a molten salt bath, but the resultant product contained higher oxide impurities.